The exchange is the latest episode between the two countries amid a mounting crisis over Tehran's nuclear programme and a shadow war marked by cyber-attacks and assassinations in Iran.

Tough new sanctions led by the US are squeezing Iran's economy while Washington is accused of staging cyber sabotage on Tehran's uranium enrichment plants.

'International airspace'

The US military drone was "never in Iranian air space" and came under fire from Su-25 fighters off the Iranian coast over international waters, Little said.

The robotic Predator aircraft was conducting "routine surveillance" and the US has told Iran it has no plans to suspend the flights, he said.

"The United States has communicated to the Iranians that we will continue to conduct surveillance flights over international waters over the Arabian Gulf consistent with longstanding practice and our commitment to the security of the region," the spokesman said.

Little said the US was prepared to safeguard its forces.

"We have a wide range of options, from diplomatic to military, to protect our military assets and our forces in the region and will do so when necessary," Little said.

The Predator returned safely to an unspecified military base in the region following the incident that occurred at 4:50am US eastern standard time (08:50 GMT).

The MQ-1 drone, a turboprop plane that flies at a much slower speed than the fighter jets, was pursued further by the Iranian warplanes but not fired on again, Little added.

The Predator was intercepted about 16 nautical miles off the Iranian coast, beyond the country's territorial waters that extend 12 nautical miles off the country's shore, the Pentagon said.

More sanctions

At the US State Department, officials unveiled more sanctions on Iran, targeting the communications minister and the culture ministry among others for censoring the media and the internet.

The move against Communications Minister Reza Taghipour came after he was blamed for ordering the jamming of international satellite TV broadcasts and restricting internet access, a department official said.

The US is determined to stop the "Iranian government from creating an 'electronic curtain' to cut Iranian citizens off from the rest of the world," said State Department spokeswoman Victoria Nuland.